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Un-bear-able (ha) predation creates variable natural selection

Natural selection is a fact of life. As Steven Jay Gould put it, it’s an “inescapable conclusion” arising from the “undeniable facts” that (1) populations of living things have inheritable variation in many traits; and (2) produce a surplus of offspring. But populations often experience selection from multiple sources, and in conflicting directions. The cover article for this month’s issue of Evolution suggests that bears may be creating ongoing selection in wild salmon populations, but the strength, and outcome, of that selection varies from stream to stream [$-a].

Salmon are famously anadromous — they hatch in freshwater streams and swim out to sea, only to return to the stream of their birth to spawn before they die. Male salmon are generally better off if they’re bigger, both to maximize stored energy for the return to their spawning site, and to better compete for mates when they arrive. Natural selection for larger bodies, however, is checked by bears, who preferentially target large, fatty fish. Yet bear predation varies from stream to stream: in narrower streams, where salmon are easier to catch, bears can fill up on big, newly-arrived fish; but in wide streams, bigger fish can more easily evade bears, so bears tend to target older, weaker fish instead.

In the new study, Carlson et al. show that this variation in the ease with which bears can catch big, newly arrived salmon results in variable natural selection among different streams. In three streams of different widths, they estimated the reproductive success of male salmon by measuring the gonads — which shrink with each mating — of males dead from old age or killed by bears. They then compared the relative reproductive success of these males with measurements of their body size. They found that the fitness of male salmon in the three study populations was a result of the compromise between the practical need to be larger, and the danger of predation by bears. More intense bear predation resulted in selection for smaller males; less intense predation favored larger males. What’s more, the analysis found directional selection, but not stabilizing selection — if this is correct, then the three salmon populations have not reached an evolutionary equilibrium yet.

What this means for these salmon populations over time is hard to say. Divergent natural selection has resulted in rapid evolution of reproductive isolation [$-a] in introduced salmon populations. However, the direction and intensity of natural selection can vary strongly over time [$-a] — so the directional selection Carlson et al. document here, over two or three years in each stream, may look very different measured over a longer sampling period.